Interface Designs And Controllable Fabrication Of Micro-silicon/carbon Anodes In Lithium-ion Batteries

The development of new high-capacity electrode materials is the key to improving the energy density and rate performance of lithium-ion batteries.Low-cost micro-silicon（Si）has both high tap density and high theoretical gravimetric and volumetric capacity,and is the most promising next-generation high-capacity anode material.However,the micro-Si anode suffers the issues of severe volume expansion and the particle pulverizations during cycling.Carbon materials are widely used in Si anodes to buffer the volume expansion,isolate the electrolytes,and improve their electronic conductivity.Nevertheless,towards the practical use of micro-Si anodes,it remains challenging to solve two problems:1)interface mismatch between the dynamic expansion of silicon and the statically buffered carbon framework and 2)the severe interface side reactions between pulverized Si and electrolytes.This dissertation focuses on the micro-Si/carbon interfaces,and proposes the strategy of building a self-adaptive dynamic micro-Si/carbon interface with pre-compressed carbon nanotubes as bridges and constructing a fluorination-enhanced flexible carbon protective layer.As a result,the transfer efficiency of ions and stress in electrodes during cycling are improved,the Si-carbon-electrolyte interface is stabilized,high volumetric capacity and excellent cyclic stability of micro-silicon anode are also achieved.As a step forward toward the industrialization of micro-Si anodes,the scalable fabrication technique of high-performance micro-Si/carbon anode materials are developed based on the self-assembly of graphenes.The main research perspectives and related contributions are summarized as follows:A self-adaptable carbon buffer structure with pre-compressed carbon nanotubes inside is produced for noncarbon nanoparticles（Sn O₂ and Si）by employing the capillary-shrinking graphene hydrogels.This obtained strong dense carbon structure can robustly buffer the volume fluctuations of noncarbons and retain a dynamic electrical connectivity with expanding/contracting noncarbon particles upon charging and discharging.With this mechanically and electrically self-adaptable carbon buffer for noncarbon particles,high volumetric capacities(up to 1920 m Ah cm^-3)together with long cycle life（up to 600 cycles）can be achieved.With an extension of this“nanospring”structure design,self-adaptable micro-Si/carbon anode material is also designed and fabricated.As a result,the“nano-spring”carbon nanotubes can conduct an electrically remedy of the broken micro-Si particles,greatly improving its cycling stability.In order to improve the mechanical and electrochemical stability of the carbon-based interface protective layer in the micro-Si anode,cobalt trifluoride（Co F₃）is used to achieve the fluoridation of the chemical vapor deposition（CVD）carbon coating on the micro-Si.In fabrication,the carbon-fluorine covalent bond is formed and the silicon component is etched to produce void space at the same time.As a result,the cycling performance and rate performance of the fluorinated CVD carbon-coated micro-silicon have been significantly improved,and the initial Coulomb efficiency（ICE）can reach90%.The profiled XPS results show that the content of lithium fluoride（Li F）in the SEI inner layer on the surface of the carbon layer has a certain increase.This carbon-based composite protective layer composed of high modulus Li F-rich SEI improves the cycling stability of micro-Si anode（achieving a stable 240 cycle life）and rate performance(delivering 1594 m Ah g^-1 at a current density of 5 A g^-1).In order to promote the practical use of micron silicon/carbon anode materials,we firstly perform a size optimization of micro-Si anode particles,and then propose the strategy of void space reservation in carbon cages combined with mechanically toughened carbon network.Furthermore,based on the liquid self-assembly and densification process of graphenes,we develop a scalable equipment and technology to fabricate dense micro-Si/carbon anode materials,and realize the fabrication of low-cost,Kg-scale micron-Si/carbon anode materials.At the same time,this anode materials delivers an excellent electrochemical performance（achieving a 100-cycle stable cycling with a 91%capacity retention under a practically high areal capacity）...